Surge arrester arranged to provide failure indication

ABSTRACT

A failure indicator device for a surge arrester (2) employs a conventional explosive disconnector (10). Operation of the disconnector (10) due to earth fault current passing through the arrester (2) causes an arc to be formed. Movement of the disconnector (10) away from the arrester (2) is guided and limited by a support arrangement (18, 20) that subsequently provides a solid and permanent conductive path to earth (6) from the arrester terminal (12), thus extinguishing the arc.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to surge arresters, and in particular toassemblies comprising a surge arrester that are arranged to provide anindication in the event of their failure.

2. Introduction to the Invention

Surge arresters, sometimes referred to as surge diverters, are devicesarranged to protect other electrical equipment, usually in thedistribution and supply of electric power, from an excessively high, andthus usually damaging, electric voltage, caused, for example, by alightning strike. The surge arrester is electrically connected betweenthe equipment at high voltage, say 5 kV or higher, and earth potential,and is electrically insulating in the absence of the overvoltage, thatis to say a voltage in excess of that which can be withstood by theassociated equipment. On occurrence of an overvoltage, the surgearrester becomes conductive so as safely to divert the consequentcurrent to earth. The arrester then reverts to its insulating condition.

In some instances, however, a lightning strike may occur so close to asurge arrester that the electrical power that flows through the arresteris so intense as to damage it, in extreme cases totally destroying it.In less extreme cases, the damage may result in the formation of acontinuous current path to earth. A surge arrester can also be subjectto other fault conditions. Faults can arise in power distributionnetworks in which a relatively low fault current, of say 10 amps, flowsfor a relatively short time between high voltage and earth through thesurge arrester. This can occur in particular in networks employingisolated neutral conductors, neutral conductors with impedancegrounding, and those with uni-grounded neutral conductors associatedwith very high grounding resistance at a remote location. In such cases,the surge arrester can fail internally but with no external evidence ofdamage, so that visual identification of the failed arrester by servicepersonnel is difficult.

Disconnectors are sometimes employed to isolate a failed arrester fromthe voltage applied thereto or from earth. The disconnector is arranged,upon detection of a predetermined fault condition, usually a flow of aminimum current for a minimum time, physically to separate from thesurge arrester, thus interrupting the path to earth. An arc is initiallydrawn, but is extinguished as the disconnector moves further away.However, unless visual inspection of every surge arrester in the networkis to be made after each temporary phase-to-ground fault, the networkwill continue to operate in a condition with a failed and disconnectedarrester, and thus at a reduced safety level. This can arise becauseearth faults on an overhead power line, for example, are fairly common.The earth fault, that is to say the operation of the disconnector, isdetected at the system control room and a circuit breaker switches offthe power supply and after a short time switches the power on again. Ifthe fault were a temporary one, the system would then continue in itspowered state. The earth fault may have arisen at any part of thesystem, not at all necessarily associated with a surge arrester. Thusthe fact that the fault has cleared provides no motivation for alinesman to be sent to check on the condition of all the surgearresters.

WO-A-93/01641 (Joslyn Corporation) discloses a high voltage surgearrester with a failed surge arrester signalling device. In thisarrangement, if the arrester fails, the disconnector disengages andprovides a visible indication that the arrester has failed, and at thesame time the connection to the surge arrester is re-established by afurther conductor, thereby to maintain a conductive path between thesurge arrester and earth potential. Upon explosive separation of thedisconnector from the arrester, the disconnector remains attached to thearrangement only by a flexible conductor, and forms no part of there-connection circuit. The further conductor, which establishes there-connection to the surge arrester, is formed from a resilient memberthat is released from tension by the departing disconnector and whichestablishes re-connection back to the surge arrester by means of itsresilience. In some circumstances, such as flow of high current, whichcan generate high repulsive mechanical forces, such resilient force maynot be sufficient to maintain good electrical connection.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a surge arresterassembly that overcomes, or at least alleviates, these disadvantages ofknown arrangements in a particularly convenient manner.

Thus, in accordance with one aspect of the present invention, there isprovided a surge arrester assembly comprising:

(a) a surge arrester having

(i) a first terminal for connection, in use, to a remote second terminalat a first electric potential, and

(ii) a third terminal for connection, in use, to a remote fourthterminal at a second electric potential, which is different from saidfirst potential, wherein, in use, the first terminal is at a locationthat is vertically lower than said third terminal;

(b) means arranged, in operation, electrically to connect the firstterminal of the surge arrester to said second terminal, said connectionmeans being arranged to be physically disconnected from the firstterminal in response to a predetermined fault condition of the surgearrester; and

(c) re-connection means arranged to provide, subsequent to saiddisconnection, a solid electrically conductive path from said firstterminal of the surge arrester to said second terminal; wherein

(d) the re-connection means is arranged to guide the connection means,upon disconnection, away from the surge arrester and to support theconnection means at a location that is vertically lower than said firstterminal.

DETAILED DESCRIPTION OF THE INVENTION

The pre-determined fault condition may be characterised by a flow ofcurrent in excess of 15 amps for a time in excess of 0.5 seconds, forexample.

Preferably, the connection means comprises an insulated elongateconductive member that is in direct electrical contact with the firstterminal of the surge arrester until disconnection takes place, andwhich, subsequent thereto, makes electrical contact with saidre-connection means.

Advantageously, upon disconnection, the connection means is guidedthrough a fifth terminal that forms part of the support structure of there-connection means and that is electrically connected to the firstterminal of the surge arrester, such that when the connection means issupported by the re-connection means, the conductive member thereof isin electrical contact with the fifth terminal.

Preferably, at least that portion of the connection means that extendsthrough the fifth terminal prior to disconnection is rigid.

Preferably, the connection means has a tapering outer circumferentialsurface and, subsequent to disconnection, is supported in a taperingaperture of the re-connection means, the respective angles of taperbeing different from one another. In such a construction, as opposed toone in which the angles of taper are substantially the same, when thefault has been repaired, the connection means can be easily removed fromthe support member of the re-connection means.

Usually, the first, lower, terminal of the surge arrester will besubstantially at earth potential, and the second terminal will be anearthing point, whilst the third, upper, terminal will be at the highvoltage (say 1 kV or above) of the fourth terminal, which may form partof electrical equipment, for example being an overhead powerdistribution conductor. Accordingly, hereinafter for convenience, and ina non-limiting manner, reference will be made to connection with respectto earth potential.

The assembly of the present invention thus provides a disconnectionfunction in the event of failure of the arrester. Separation of theearth connecting means from the earthing terminal of the arrester thenleads to formation of an arc. The subsequent provision of a solid, thatis to say non-gaseous, and permanent conductive path to earth from theterminal causes the arc to be extinguished and the fault current to flowto earth along that path. It is particularly advantageous that it is theearth connecting means itself that not only responds to the fault bybecoming physically disconnected from the surge arrester terminal butthen subsequently forms part of the solid path to earth. The fact thatthe fault current is maintained even though the arc produced byinterruption of the earth connecting means has been extinguished, willprevent the associated circuit breaker permanently switching back on thepower supply to the equipment. Thus, a linesman must then be sent toidentify and replace the failed arrester, before power can be restored,ensuring the safety of the electrical system.

Preferably, the earth connecting means is movable, in response to saidfault condition, between a first position in which it is physicallyattached to the surge arrester and a second position in which it issupported on a member spaced apart from said surge arrester terminal.Usually when a conventional disconnector is employed, it hangs freelydown from the surge arrester after its operation, as disclosed inWO-A-93/01641. By arranging for the disconnection to take place from thelower terminal of the surge arrester, the force of gravity will ensureeffectiveness of the separating movement. Furthermore, by guiding theconnection means to a specific support member as it disconnects, it canbe ensured that the arc drawn between the first surge arrester terminaland the receding disconnector is properly extinguished. The subsequentpositive support and retention of the disconnector enhances theintegrity of the re-established, solid conductive path and minimises thedanger of a free-moving disconnector accidently reestablishing an arc tothe surge arrester terminal.

It is to be understood that the first terminal of the surge arrester inbeing located vertically lower than the third terminal, need not bedirectly vertically below it. That is to say, the orientation of thesurge arrester need not be substantially vertical, although this may beits conventional orientation, but it may be inclined to the vertical,with disconnection being arranged to take place from the lower of itstwo terminals. In the event that the arrester is mounted substantiallyhorizontally, disconnection may take place from either of its terminals,and either one may then be regarded as being at a location that isvertically lower than the other.

The support member advantageously is electrically connected to thearrester terminal, so as to provide the solid conductive path.

Preferably, at least a portion of the earth connecting means is rigidand is guided by the support member in its movement between said twopositions. Advantageously, the rigid portion of the earth connectingmeans comprises an insulated conductive elongate member that extendsthrough an aperture in the support member.

The surge arrester itself may be of any suitable construction. Forexample, the surge arresting elements may comprise a plurality ofvaristor blocks, comprising zinc oxide material for example. The surgearresting elements are contained within an outer insulating housing,which is preferably formed of polymeric material, which may be heatshrunk into place. The housing may be provided with one or more sheds,so as to enhance the length of the earth leakage path between theterminals of the arrester and to shed liquid pollution therefrom.

Surge arrester assemblies, in accordance with the present invention,will now be described, by way of example, with reference to theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an elevation of an assembly in its normal passiveconfiguration;

FIG. 2 shows the assembly of FIG. 1 just after a fault has occurred;

FIG. 3 shows the assembly of FIG. 1 in its final post-faultconfiguration,

FIGS. 4A and 4B shown schematically in enlarged detail a furtherassembly having a modified disconnection arrangement respectively beforeand after operation;

FIGS. 5A and 5B show a still further assembly having adifferently-modified disconnection arrangement;

FIGS. 6A and 6B show a yet further assembly having adifferently-modified disconnection arrangement; and

FIG. 7 shows an enlarged detail of a further modification of theassembly of FIGS. 1 to 3.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, a shedded polymeric surge arrester 2 has anupper terminal 4 connected to remote second terminal 5 which may be partof high voltage equipment (not shown). At its lower end, the arrester 2is physically supported by an earthed block 6, being spaced therefrom byan insulator 8. A disconnector 10 is mounted at the lower end of thearrester 2 in electrical connection with the lower, earthing, terminal12 of the arrester (FIGS. 2, 3). A rigid insulated conductive rod 14extends down from a terminal 15 of the disconnector 10, which isconnected to the terminal 12, and is connected by a flexible conductor16 to the earthed block 6, thereby to provide earthing for the arresterterminal 12.

An insulated conductive bracket 18 is mounted at the lower end of thesurge arrester 2 in electrical connection with the earthing terminal 12.The bracket 18 extends down from the arrester 2 in an L-shape so as todispose an electrode 20 directly beneath the disconnector 10. Theinsulated rod 14 extends through an aperture in the electrode 20.

As indicated by FIG. 1, the high voltage equipment (not shown) connectedto the high voltage terminal 4 of the surge arrester 2, and the surgearrester 2 itself, are functioning normally. That is to say, thearrester 2 is acting as an insulator, and no current is flowing to earthat the block 6.

In the event of a fault in the arrester 2, a fault current flows throughthe surge arrester 2, is sensed by the disconnector 10 as it passestherethrough, and then to earth at the block 6 via the rod 14 and theflexible conductor 16. The disconnector 10 operates to eject itself awayfrom the surge arrester (FIG. 2). An electric arc 22 is drawn betweenthe earthing terminal 12 of the surge arrester and the recedingdisconnector 10. The disconnector 10 is guided in its movement by thesliding passage of the rigid insulated rod 14 through the electrode 20on the supporting bracket 18.

Movement of the disconnector 10 is stopped by its engagement with thebracket 18 (FIG. 3). In this position, good electrical contact isestablished between the lower terminal 15 of the disconnector 10 whichfits into the bracket electrode 20. A solid and permanent electricallyconductive path is thus now established from the earthing terminal 12 ofthe surge arrester, through the support bracket 18 to its electrode 20,then via the disconnector terminal 15 and insulated rod 14 and flexibleconductor 16 to the earthed block 6. Since the impedance of this path isless than that to earth through the arc 22, the arc is extinguished andall the fault current passes along the solid path.

The continuous flow of current through the surge arrester to earth willnow prevent the circuit breaker associated with the high voltageequipment and its power supply from re-establishing the supply of power.The failed surge arrester will thus have to be replaced, thusmaintaining the electrical safety of the system.

FIGS. 4A and 4B show a slightly modified form of disconnectionarrangement 10', in which its lower terminal 15' is of frusto-conicalshape to provide a stable mating engagement within the bracket electrode20. FIG. 4B shows the disconnector 10' after it has operated, with anupper part 24 still attached to the surge arrester 2 and the lower part26 spaced therefrom and supported on the bracket 18. The insulation ofthe rod 14 is partially cutaway to reveal the enclosed conductivemember.

FIGS. 5A and 5B show a modification of the disconnector 10' of FIGS. 4Aand 4B, in which an insulating, or insulated, helical coil 28 is shownschematically located between the surge arrester 2 and the lower arm ofthe bracket 18 so as to act as a guide for the movement of the rod 14and lower part 26 of the disconnector as it separates from its upperpart 24. The open structure of the guide 28 allows any fragments fromthe disintegrating disconnector 10' to pass therethrough.

FIGS. 6A and 6B show a further modification of the disconnectionarrangement in which an insulating vented tube 40 depends downwards fromthe surge arrester 2 and supports the electrode 20. The insulated rod 14extends upwardly out of the bracket electrode 20, as can be seen in thecutaway portion of the tube 40, and is guided in a channel 42 of thetube 40. The channel 42 and other channels 44 are vented to the exteriorto allow explosive gases to pass out of the tube 40 on occurrence of thedisconnection.

Referring to FIG. 7, the enlarged detail of a modification of theassembly 2 in its disconnected state (FIG. 3) mounted on a supportstructure, shows the lower end of a disconnector 10a, its terminal 15aand a frustoconical conductive portion 50 of cone angle β extendingtherefrom, the portion 50 leading to a rigid insulated conductive rod14a. The support bracket is provided with an electrode 20a in the formof a frustoconical aperture 52 of cone angle α. The configurations arearranged so that α≠β, and, as shown, α>β, whereby only line contactexists between the components 50, 52 rather than the surface contactthat would exist if α=β. Subsequent separation of the disconnector 10afrom the support bracket is thus facilitated.

What is claimed is:
 1. A surge arrester assembly comprising(a) a surgearrester having(i) a first terminal for connection, in use, to a remotesecond terminal at a first electric potential, and (ii) a third terminalfor connection, in use, to a remote fourth terminal at a second electricpotential, which is different from said first potential, wherein, inuse, the first terminal is at a location that is vertically lower thansaid third terminal; (b) connection means arranged, in operation,electrically to connect the first terminal of the surge arrester to saidsecond terminal, said connection means(i) being arranged to bephysically disconnected from the first terminal in response to apredetermined fault condition of the surge arrester, and (ii) comprisingan insulated elongate conductive member that is in direct electricalcontact with the first terminal of the surge arrester untildisconnection takes place; and (c) re-connection means arranged toprovide, subsequent to said disconnection, a solid electricallyconductive path from said first terminal of the surge arrester to saidsecond terminal; wherein(1) the re-connection means is arranged to guidethe connection means, upon disconnection, away from the surge arresterand to support the connection means at a location that is verticallylower than said first terminal, (2) the connection means, subsequent todisconnection, makes electrical contact with said re-connection means,and (3) upon disconnection, the connection means is guided through afifth terminal that forms part of the support structure of there-connection means and is electrically connected to the first terminalof the surge arrester, such that when the connection means is supportedby the re-connection means, the conductive member thereof is inelectrical contact with the fifth terminal.
 2. An assembly according toclaim 1, wherein at least that portion of the connection means thatextends through the fifth terminal prior to disconnection is rigid. 3.An assembly according to claim 2, wherein the connection means has atapering outer circumferential surface and, subsequent to disconnection,is supported in a tapering aperture of the re-connection means, therespective angles of taper being different from one another.
 4. Anassembly according to claim 2, wherein said second terminal issubstantially at earth potential and said fourth terminal forms part ofelectrical equipment at high voltage, whereby the connection means isarranged to connect the first terminal of the surge arrester to earthpotential.
 5. An assembly according to claim 2, wherein the surgearrester comprises a plurality of varistor blocks disposed within aninsulating housing.
 6. An assembly according to claim 5, wherein theouter surface of the surge arrester housing is of shedded configuration.7. An assembly according to claim 1, wherein the connection means has atapering outer circumferential surface and, subsequent to disconnection,is supported in a tapering aperture of the re-connection means, therespective angles of taper being different from one another.
 8. Anassembly according to claim 1, wherein said second terminal issubstantially at earth potential and said fourth terminal forms part ofelectrical equipment at high voltage, whereby the connection means isarranged to connect the first terminal of the surge arrester to earthpotential.
 9. An assembly according to claim 1, wherein the surgearrester comprises a plurality of varistor blocks disposed within aninsulating housing.
 10. An assembly according to claim 9, wherein theouter surface of the surge arrester housing is of shedded configuration.11. A surge arrester assembly comprising(a) a surge arrester having(i) afirst terminal for connection, in use, to a remote second terminal at afirst electric potential, and (ii) a third terminal for connection, inuse, to a remote fourth terminal at a second electric potential, whichis different from said first potential, wherein, in use, the firstterminal is at a location that is vertically lower than said thirdterminal; (b) connection means arranged, in operation, electrically toconnect the first terminal of the surge arrester to said secondterminal, said connection means(i) having a tapering outercircumferential surface and (ii) being arranged to be physicallydisconnected from the first terminal in response to a predeterminedfault condition of the surge arrester; and (c) re-connection meansarranged to provide, subsequent to said disconnection, a solidelectrically conductive path from said first terminal of the surgearrester to said second terminal; wherein(1) the re-connection means isarranged to guide the connection means, upon disconnection, away fromthe surge arrester and to support the connection means at a locationthat is vertically lower than said first terminal, and (2) subsequent todisconnection, the connection means is supported in a tapering apertureof the re-connection means, the respective angles of taper beingdifferent from one another.
 12. An assembly according to claim 11,wherein the connection means comprises an insulated elongate conductivemember that is in direct electrical contact with the first terminal ofthe surge arrester until disconnection takes place, and which,subsequent to disconnection, makes electrical contact with there-connection means.
 13. A surge arrester assembly comprising(a) a surgearrester having(i) a first terminal for connection, in use, to a remotesecond terminal at a first electric potential which is substantially atearth potential, and (ii) a third terminal for connection, in use, to aremote fourth terminal at a second electric potential, which isdifferent from said first potential, wherein (1) in use, the firstterminal is at a location that is vertically lower than said thirdterminal, and (2) said fourth terminal forms part of electricalequipment at high voltage; (b) connection means arranged, in operation,electrically to connect the first terminal of the surge arrester to saidsecond terminal at earth potential, said connection means(i) beingarranged to be physically disconnected from the first terminal inresponse to a predetermined fault condition of the surge arrester, and(ii) comprising an insulated elongate conductive member that is indirect electrical contact with the first terminal of the surge arresteruntil disconnection takes place; and (c) re-connection means arranged toprovide, subsequent to said disconnection, a solid electricallyconductive path from said first terminal of the surge arrester to saidsecond terminal; wherein(1) the re-connection means is arranged to guidethe connection means, upon disconnection, a from the surge arrester andto support the connection means at a location that is vertically lowerthan said first terminal, and (2) the connection means, subsequent todisconnection, makes electrical contact with said re-connection means.14. A surge arrester assembly comprising(a) a surge arrester(i) having afirst terminal for connection, in use, to a remote second terminal at afirst electric potential, (ii) having a third terminal for connection,in use, to a remote fourth terminal at a second electric potential,which is different from said first potential, wherein, in use, the firstterminal is at a location that is vertically lower than said thirdterminal, and (iii) comprising a plurality of varistor blocks disposedwithin an insulating housing; (b) connection means arranged, inoperation, electrically to connect the first terminal of the surgearrester to said second terminal, said connection means(i) beingarranged to be physically disconnected from the first terminal inresponse to a predetermined fault condition of the surge arrester, and(ii) comprising an insulated elongate conductive member that is indirect electrical contact with the first terminal of the surge arresteruntil disconnection takes place; and (c) re-connection means arranged toprovide, subsequent to said disconnection, a solid electricallyconductive path from said first terminal of the surge arrester to saidsecond terminal; wherein(1) the re-connection means is arranged to guidethe connection means, upon disconnection, away from the surge arresterand to support the connection means at a location that is verticallylower than said first terminal, and (2) the connection means, subsequentto disconnection, makes electrical contact with said re-connectionmeans.
 15. An assembly according to claim 14, wherein the outer surfaceof the surge arrester housing is of shedded configuration.